Lyme disease is caused by the spirochete Borrelia burgdorferi (Bb) in North America and predominantly by Borrelia afzelii and Borrelia garinii in Eurasia. The pathogen is transmitted by the bite of a tick (Ixodes sp.), deer tick or western black-legged tick in North America. One of the typical first signs of Borrelia sp. infection is erythema migrans (EM), a bull's eye-like skin lesion, which arises within a few days of the bite. However, the bite may not be painful, and a rash does not develop in all patients. Patients can also experience flu-like symptoms, however, such symptoms may be attributed by the patient to any of a number of other causes.
After infection, the spirochetes can disseminate in the bloodstream to various target tissues. Prompt treatment with antibiotics can typically kill the pathogen and prevent long term ill effects of infection. If the infection is not treated, or not successfully treated, it can result in neurological, rheumatological, and cardiac damage over time. Some common symptoms from long-term Lyme infection include arthritis, facial palsy, and neuroboreliosis. Even after antibiotic treatment of Lyme disease, some individuals show post-Lyme disease syndrome (PLDS) and have lingering symptoms such as fatigue, musculoskeletal pain, and cognitive complaints.
Because of the difficulty in culturing Borrelia bacteria in the laboratory, diagnosis of Lyme disease is typically based on clinical exam findings and a history of exposure to endemic Lyme areas (Ryan K J, Ray C G (editors) 2004. Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 434-437). The EM rash, which does not occur in all cases, is considered sufficient to establish a diagnosis of Lyme disease even when serologic blood tests are negative (Hofmann et al., 1996, Infection 24: 470-472; Pachner et al., 1989. Rev. Infect. Dis. 11 Suppl 6: S1482-1486). Serological testing can be used to support a clinically suspected case but is not diagnostic by itself (Ryan K J, Ray C G (editors) 2004. Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 434-437).